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Wideband Frequency Demodulator

IP.com Disclosure Number: IPCOM000092939D
Original Publication Date: 1967-Mar-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 55K

Publishing Venue

IBM

Related People

Belleson, JG: AUTHOR

Abstract

This frequency demodulator incorporates a generator of a pulse of constant polarity, height and width at each zero crossing of the FM signal. Compensation for nonlinearity or mismatch of components is included.

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Wideband Frequency Demodulator

This frequency demodulator incorporates a generator of a pulse of constant polarity, height and width at each zero crossing of the FM signal. Compensation for nonlinearity or mismatch of components is included.

FM source 10 feeds a series of AC coupled limiters 12. The latter transform FM signal A to pulse train B in which the positive-going transitions correspond to the positive-going zero crossing of the FM signal and the negative-going transitions correspond to the negative-going zero crossings.

Detector 14 generates a constant width, constant height, positive pulse for each transition of signal B. Shorted delay line 16, which has a source impedance equal to its characteristic impedance, generates a positive pulse for each positive-going step function and a negative pulse for each negative-going step function. The width of each of these pulses is the same, for it is determined by the delay of line 16, chosen so that the pulse width is less than the minimum time between transitions in signal B. Transistor T18 isolates line 16. Resistors R20 and R22 are selected so that the Thevenin source impedance seen by line 16 equals its characteristic impedance. The contribution of the input capacitance of T18 to the source impedance is greatly reduced by R22. Resistors R24, R26, R28, and R30 are large and operate to provide constant current generators.

The base biases of transistors T32 and T34 differ by the voltage drop across resistor R38. Thus the former is cut off while the latter is conducting, if there is no signal at the emitter of T18.

A positive pulse at the emitter of T18 propagates through diode D42 turning T32 on and T34 off. A n...